Perhaps other mixers desperate to destroy the competition? There are some with a dodgy reputation who are probably struggling to lure victims. Then you have those offering genuine and legitimate mixing services but can't make ends meet with the low turnover from the few users mixing coins on their sites.
Then of course you have those malicious actors who take pleasure in destroying other people's work or who resent their success.
Envy knows no bounds. People filled with it will do things that cross the bounds of human decency.